1. Field of the Invention
The present invention relates to a digital watermark; more specifically, the invention relates to a method of providing hidden digital watermarks in images.
2. Description of Related Art
Due to the rapid and extensive growth of electronic publishing industry, issues regarding protection of copyright have become increasingly important, in a large part because digital data in media form is easily copied and manipulated. One potential solution for claiming the ownership of a digital image is to use electronic stamps or so-called watermarks, which are embedded into the images, wherein the watermarks containing the copyright information should be added to the original digital image data without being perceptually visible to the viewers.
In order to achieve the above objects, conventional technologies provide an image authentication technique by embedding digital watermarks into original images. Details regarding technologies of this sort, which are still being developed, are disclosed in xe2x80x9cElectronic marking and identification techniques to discourage document copyingxe2x80x9d by J. T. Brassil, S. Low, N. F. Maxemchuk, and L. O""Gorman, pp. 1495-1504, IEEE J. Select. Areas Commum., vol. 13, October 1995; there different encoding methods are mentioned in the document to conceal digital markings in a document. Furthermore, related publications including xe2x80x9cApplying signatures on digital imagesxe2x80x9d by I. Pitas and T. H. Kaskalis, pp. 460-463, in Proc. IEEE Nonlinear Signal and Image Processing, June 1995; xe2x80x9cSpatial method for copyright labeling of digital imagesxe2x80x9d by O. Bruyndonckx, J. J. Quisquater, and B. Macq, pp. 456-459, in Proc. IEEE Nonlinear Signal and Image Processing, June 1995; xe2x80x9cImage authentication for a slippery new agexe2x80x9d by S. Walton, pp. 18-26, Dr. Dobb""s Journal, April 1995; and xe2x80x9cTechniques for data hidingxe2x80x9d by W. Bender, D. Gruhl, and N. Morimoto, p. 40, Proc. SPIE, vol. 2420, February 1995 are conventional technologies to provide digital watermarks in spatial domain.
However, while technologies for concealing added information in digital data are readily available, it is difficult to make sure that the digital watermarks survive common digital image processing operations such as image compression (as in JPEG format), change of geometry (for example, image cropping, ornamental change, etc.), D/A-A/D converting (for example, scanning on a printout), noise interference, and so forth, with the biggest challenge of all being maintaining the watermark on a reduced image after a lossy data compression that eliminates secondary information. A technology for providing digital watermarks in a frequency domain, which superimposes a copyright code and its random sequence of locations for embedding on the image based on a JPEG model, is also readily available, as disclosed in xe2x80x9cToward robust and hidden image copyright labelingxe2x80x9d by E. Koch and J. Zhao, pp. 452-455, in Proc. IEEE Nonlinear Signal and Image Processing, June 1995, xe2x80x9cSecure spread spectrum watermarking for multimediaxe2x80x9d by I. J. Cox, J. Kilian, T. Leighton, and T. Shammoon, in Tech. Rep. 95-10, NEC Res. Inst., Princeton, N.J., 1995, and xe2x80x9cTransparent robust image watermarkingxe2x80x9d by M. D. Swanson, B. Zhu, and A. H. Tewfik, pp. 211-214, in Proc. ICIP ""96, etc.
However, the above-mentioned conventional technologies do not take into consideration the embedding of the digital watermarks with visible recognizable patterns such as signatures in the forms of Chinese characters and personal seals.
In order to improve on the aforementioned technical shortcomings and inadequacies, the present invention is related to a method of providing hidden digital watermarks in images, comprising: providing an original image and a watermark image; applying pseudo-random permutations to the watermark image for generating a dispersed watermark image; applying block-based permutations to the original image and the dispersed watermark image in order to form a plurality of original image blocks with each of the watermark blocks dispersed over the corresponding image block only; applying FDCT (Forward Discrete Cosine Transform) on each of the original image blocks independently so that each of the original image blocks is transformed into a DCT coefficient block that corresponds to different frequency ranges; embedding said watermark image blocks into said DCT coefficient blocks, which are the original image blocks of specified frequency range, in order to form a plurality of combined DCT coefficient blocks; applying IDCT (Inverse Discrete Cosine Transform) to the combined DCT coefficient blocks to form an embedded watermark image.